Saturday, 23 October 2010

Not that old chestnut again!

A couple of decades ago, we had the Dutch Elm disease problem that decimated our native Elm trees. In fact I am told that there are only about a couple of hundred Elm trees left in the UK. Dutch Elm disease is a  fungal disease of Elm which is spread by a beetle. Although believed to be originally native to Asia, the disease has been accidentally introduced into North America and Europe, where it has devastated native populations of Elms which had not had the opportunity to evolve any resistance to the disease.

Elm was a favoured wood in the past for use to build and bottom many wooden narrow boats. Often giving a working  life of twenty to thirty years between replacement.

Now with a strong feeling of Déjà vu it veems to be happening all over again. but this time to another of our native trees.

It seem to be the turn of our native Horse Chestnut tree. In the UK Horse Chestnuts have suffered increased levels of attack from the horse chestnut leaf miner, a tiny moth larve that burrows into the leaf structure. Despite the poor appearance of horse-chestnut trees infested, there is no evidence that damage by the moth leads to a decline in tree health or tree death. Trees survive repeated infestations and re-leaf normally in the following year.

Now there is also a second fungal pathogen similar to Dutch Elm disease causing stem bleeding, commonly known as bleeding canker. When the infection spreads round the full girth of the tree, the tree will die.

All of the above comes after sustained damage from a couple of hundred years of producing acid rain which is as a result of air pollution. When any type of fuel is burnt, lots of different chemicals are produced. The smoke that comes from a fire or the fumes that come out of a car exhaust don't just contain the sooty grey particles that you can see - they also contains lots of invisible gases that can be even more harmful to our environment.

Until relatively recently air pollution has been seen as a local issue. It was in southern Scandinavia in the late 1950's that the problems of acid rain were first observed and it was then that people began to realise that the origins of this pollution were far away in Britain.

Acid rain was slowly killing trees in the north of England, parts of Scotland and across in Scandinavia as a result of our coal and gas fired power stations. However, factories and cars all burn fuels and so they also produce polluting gases. Some of these gases especially nitrogen oxides and sulphur dioxide react with the tiny droplets of water in clouds to form sulphuric and nitric acids. The rain from these clouds then falls as very weak acid - which is why it is known as "acid rain".

Acid rain can be carried great distances in the atmosphere, not just between countries but also from continent to continent.  The rain sometimes falls many hundreds of miles from the source of pollution but wherever it falls it can have a serious effect on soil, trees and water. In the 1970s the effects of acid rain were seen at their worst. Forests all over the world were dying... in Scandinavia the fish were dying; lakes were crystal clear but contained no living creatures or plant life. Many of Britain's freshwater fish were threatened their eggs were damaged and deformed fish were being hatched . This in turn led to to fish-eating birds and animals also being affected.

However, all acid rain is not man made, the release of sulphur dioxide can also occur naturally, such as when a volcano erupts. Nature has mechanisms to cope with natural events - todays polution levels are much more sustained than in natural events.

Now, in some new research that has just been released, it would seem that our two tree species are just the tip of the iceberg. More than a fifth of the world's plant species faces the threat of extinction, a trend with potentially catastrophic effects for life on Earth. Stephen Hopper, director of the Royal Botanic Gardens in Kew said "the report on plant loss was the most accurate mapping yet of the threat to the planet's estimated 380,000 plant species". At the launch of the Sampled Red List Index, Hopper also said "This study confirms what we already suspected, that plants are under threat and the main cause is human-induced habitat loss," The study was carried out by Kew with the Natural History Museum in London and the International Union for the Conservation of Nature.


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